They are talking about on an iOS device, not iTunes. There is supposed to be a toggle under Settings > Music to turn iTunes Match on or off. I assume this appears only if your Apple ID is "match enabled", which mine is.
This settings toggle is gone, for some reason. I've only installed iOS 5 on my iPad 2. It'll be interesting to see if it works on the preinstalled iOS 5 on the iPhone 4S.
BTW, this appears to be a common problem. There is a thread on it in the Apple iOS 5 Beta forum (iCloud section).
-David